Some materials when cooled below a certain critical temperature (Tc) become superconducting. At the particular Tc of the material, the electrons in the material that are responsible for conduction undergo a collective transition to an ordered state having many unique and remarkable properties. These properties include the loss of resistance to the flow of an electrical current, the appearance of unusual magnetic effects such as a large diamagnetism, substantial alteration of thermal properties and the occurrence of quantum effects otherwise observable only at the atomic and subatomic level.
Some twenty six of the metallic elements are know to be superconductors in their normal forms and another ten become superconductive under pressure or when prepared in the form of highly disordered thin films. Typically, such materials become superconductive only at very low temperatures, such as the boiling point of liquid helium, which is 4.2 K. These superconductors have been called low temperature superconductors. However, it has recently been discovered that sintered materials comprising oxides of the elements of group IIa or IIIa of the periodic table can act as superconductors at higher temperatures, such as the temperature of boiling liquid nitrogen (77 K). Superconductors based on such materials have been called high temperature superconductors.
There are many potential applications for superconductors, including, but not limited to, magnets for high energy physics applications, rotating machinery (i.e., synchronous generators, homopolar d-c machines), fusion magnets, magnetodynamic generators and magnets for nuclear magnetic resonance imaging, which is also called magnetic resonance imaging. Other applications include motors for marine propulsion and levitated trains for high speed transportation.
To effectively use superconductors in certain applications, superconducting wires must be made. Typically, superconducting wires are made with a metal sheath surrounding a superconducting core. Once a superconducting wire has been made, it is desirable to coat the wire with a dielectric composition. The coating, in addition to providing better structural integrity and protection from environmental stress, insulates wires from each other, particularly when wires are used in windings for motors, magnets and the like.
A coating for a superconducting wire must, however, possess certain properties. For example, the coating must be easy to apply and cure. Preferably, the coating and cure are carried out at ambient temperature. The coating composition should produce or contain a minimum amount of volatile organic compounds that may be emitted into the atmosphere, and the coating composition should be amenable to high speed production operations, and thus have high cure speeds. Lastly, the coating must be able to withstand the temperatures to which the superconducting wire will be subjected. Typically, a superconducting wire is cycled from ambient temperature to the Tc of the superconductor by introducing the superconductor into an environment having the temperature of the boiling point of liquid helium if the superconductor is a low temperature superconductor or the temperature of the boiling point of liquid nitrogen if the superconductor is a high temperature superconductor. Because the difference between the ambient temperature and the Tc is generally very large, the coating must be able to withstand such thermal cycling without detaching from the wire, cracking, splitting or failing in any other way that would affect the insulative or protective functions of the coating.
Thermal cycling can generate mechanical stress in coating compositions because of the differences in thermal expansion between the coating and the metal sheath and superconductor, and most organic polymers, the primary components of many coating compositions, are very brittle at the critical temperatures of both high and low temperature superconductors.
The ability of a coating to retain its integrity during such thermal cycling has been a nemesis to researchers attempting to find suitable coating compositions for superconducting wire. Further, thermally cured coating compositions generally are not preferred for high temperature superconductors because the temperatures required to cure the coatings can reduce the amount of current that can be carried in a wire before losing its superconductive properties. Thus, it is preferable to avoid heating superconductors.
The present invention provides an organic coating composition for superconducting wire that can be cured at ambient temperature and which will withstand the thermal cycling process that is necessary to reach the superconductor's critical temperature.